Field of the invention
This invention relates to apparatus for lowering the level of turbulence in a cavity over which air is flowing.
Prior art
It is sometimes desirable to precisely determine, from a moving aircraft, the precise direction of an object or a source of light. Inasmuch as it is not practical to mount imaging equipment on the outside of the hull of an aircraft, this equipment is mounted inside the aircraft adjacent to a window or opening through which the light from the source passes. To obtain the most precise detection of the direction of the light source and the sharpest focus of the image of the light source, a window must be free of any glass or other cover. The reason for this is that it is not at present possible to manufacture a transparent closure for the opening which will not shift the apparent direction of the light source, and in addition, defocus the image of the light source. Thus, an open hole in the wall of the aircraft must be used.
The use of the open hole in the wall of the aircraft creates another problem. Air flowing over the hole or cavity at subsonic velocities creates a very high turbulence in the cavity or hole in the wall of the aircraft. This causes the image of the light to jump from place to place on that element of the sensing unit on which the light falls. When this happens an accurate sensing of the true direction of the light source cannot be carried out. Further, the turbulence in the hole in the aircraft wall will scatter the light rays from the source so that a sharp image of the source cannot be obtained. This, too, will result in an inaccurate determination of the true direction of the light source.
Further, the turbulence buffets equipment, such as sensors etc., mounted in the aircraft near the hole. Vibration of this equipment, caused by turbulence, will make it impossible to accurately sense the true direction of the object being observed.
It is known to use a fence to lower turbulence in an opening in a wall along which a stream of air is flowing. The fence, which is usually raked, is secured to the wall adjacent to the upstream edge of the hole. Imperforate fences lower turbulence in the opening but, to be effective, must protrude an undesirable distance into the air stream. It is also known to use a fence which is provided with apertures across the entire area of the fence. This apertured fence is superior to the imperforate fence but is still not entirely satisfactory.